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  • Determination of Plasticity following Deformation and Welding of Austenitic Stainless Steel Acar, MO & Fitzpatrick, ME Author post-print (accepted) deposited by Coventry University’s Repository Original citation & hyperlink:

    Acar, MO & Fitzpatrick, ME 2017, 'Determination of Plasticity following Deformation and Welding of Austenitic Stainless Steel' Materials Science and Engineering: A, vol 701, pp. 203-213

    DOI 10.1016/j.msea.2017.06.074 ISSN 0921-5093 Publisher: Elsevier NOTICE: this is the author’s version of a work that was accepted for publication in Materials Science and Engineering: A. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Materials Science and Engineering: A, [701, (2017)] DOI: 10.1016/j.msea.2017.06.074 © 2017, Elsevier. Licensed under the Creative Commons Attribution- NonCommercial-NoDerivatives 4.0 International Copyright © and Moral Rights are retained by the author(s) and/ or other copyright owners. A copy can be downloaded for personal non-commercial research or study, without prior permission or charge. This item cannot be reproduced or quoted extensively from without first obtaining permission in writing from the copyright holder(s). The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the copyright holders. This document is the author’s post-print version, incorporating any revisions agreed during the peer-review process. Some differences between the published version and this version may remain and you are advised to consult the published version if you wish to cite from it.

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    Determination of Plasticity following

    Deformation and Welding of Austenitic Stainless


    M. O. Acar1,3, M. E. Fitzpatrick2,3

    1Now at: Siemens Energy Inc., Hamilton, New Jersey 08638, USA

    2Centre for Manufacturing and Materials Engineering, Coventry University, Priory

    Street, Coventry CV1 5FB, UK

    3 Previously at: Department of Engineering and Innovation, The Open University,

    Walton Hall, Milton Keynes MK7 6AA, UK


    Intergranular strain has been associated with high-temperature cracking of welded

    pipework in 316H austenitic stainless steel material used in nuclear power plant heat

    exchangers. In this study, neutron diffraction has been used to study the

    development of intergranular strains in plastically-deformed and welded 316H

    stainless steel. Measurements have been made of the intergranular strain evolution

    with increasing plastic strain in base material, and correlated with further

    measurements made in samples extracted from welded pipes, where the pipes were

    welded following plastic deformation to different levels of plastic strain. Strong

    tensile strain evolution was seen on the compliant 200 grain family. The results were

    Manuscript Click here to view linked References

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    correlated with various proxy measures of plastic strain, including hardness and

    diffraction peak width, and excellent agreement was obtained.

    1. Introduction

    In power generation plants, energy is transferred to the power generator turbines via

    heat exchanger units which are exposed to high temperatures and pressures. Many

    metres of austenitic stainless steel tubes are bent, swaged and welded to produce

    these heat exchanger units. During tube-shaping and welding operations, plastic

    deformation takes place. This influences in-service performance of the material unless

    the effect of plastic deformation is fully annealed out. The ASME Boiler and Pressure

    Vessel Design code [1] specifies that the limit of forming strain for 316H stainless steel

    material which is going to operate between 580°C and 675°C is 20%, and if it exceeds

    that threshold the material should be annealed at a minimum of 1040°C to restore its

    mechanical properties, tensile strength, creep strength and creep ductility to the start-

    of-life values. Current construction practices in the UK for 316H tubes forbid welding

    onto material that has experienced more than 15% plastic strain, without first

    resolution heat treating the material.

    Although the standard codes and construction practices provide some

    guidance, it is sometimes not possible to follow these rules strictly, especially when

    the whole boiler is constructed as a large and complex single unit containing different

    tubing materials. Therefore, tubing material in-service is likely to retain some degree

    of prior plastic deformation arising from tube-shaping and welding operations.

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    Plastic deformation is considered as one of the major factors in initiating stress

    corrosion cracking [2]–[6] and creep failure [7]–[11] of austenitic stainless steels.

    Creep resistance increases with plastic strain, whereas creep ductility drastically

    decreases [10].

    There is no direct method to accurately determine the degree of plastic

    deformation to which a material was previously subjected. Hardness measurement

    and X-ray diffraction peak broadening are used for the qualitative determination of

    the plastic strain. However, there have been recent promising attempts at the

    development of experimental methods to measure plastic deformation, based on the

    observation of crystallographic changes in materials after plastic deformation using

    neutron diffraction [12], [13] and electron back-scattered diffraction (EBSD) [14].

    Elastic anisotropy means that the elastic stiffness of a crystal is dependent on

    its orientation relative to the applied load. The yield stress may also depend on the

    orientation of the crystal, which makes the material plastically anisotropic. Because of

    elastic and plastic anisotropy, austenitic stainless steel has a highly complex stress-

    strain response [15].

    When a material elongates under uniaxial load, the slip direction will tend to

    rotate towards the loading direction, which means that the grains reorient with

    increasing plastic deformation. When the load is released, residual strains develop: a

    combination of residual intergranular strains (Type II) and residual intragranular

    strains (Type III) [16]. Intergranular strains self-equilibrate over a length scale of the

    order of the grain size. They develop owing to elastic anisotropy of the grain and the

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    constraint of neighbouring grains. Intragranular strains are generated over a length

    scale smaller than the grain size. Their origin is generally crystal defects such as

    dislocations, solute atoms and vacancies. Hereafter, we focus on the type II

    intergranular strains and will use the term “residual intergranular strains” as

    previously used in the literature [17], [18].

    Another effect of plastic deformation is the increase in dislocation density and

    the formation of dislocation bands and arrays. Severe cold working of an annealed

    metal will increase the dislocation density from around 107 to 1011 dislocations/cm2

    [19]. In a grain, as the deformation proceeds, dislocations intersect each other and

    start to lose their mobility, which explains the strain hardening process. With further

    plastic deformation they begin to condense into bands and arrays. These structures

    can be considered as low-angle boundaries (LAB) within the grains, where the

    separation between two crystallographic orientations is only a few degrees, less than

    in a high-angle grain boundary (>15°) [19].

    Diffraction techniques enable tracking of the changes in residual intergranular

    strains and the dislocation density with increasing plastic deformation. Neutron

    diffraction is particularly attractive as it has good penetration into metallic alloys (e.g.

    >50 mm in steel) and is able to measure a defined volume inside a bulk component

    [20]. The diffraction peak position obtained for an hkl plane is shifted depending on

    the accumulated strain on this plane. The accumulated strain on each plane is

    different owing to the differences in elastic and plastic anisotropy, and therefore, the

    peak shift changes for individual planes. By using the peak shifts in a diffractogram,

    Pang et al. measured the residual intergranular strains developed after unloading

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    from 8% uniaxial straining of a rolled 309H stainless steel in the rolling direction [21].

    They constructed residual strain pole figures for the 111, 200, 331 and 224 planes and

    found that high tensile residual strains develop on the 200 planes in the direction of

    straining parallel to the rolling direction. Peng et al. also measured residual

    intergranular strains at different specimen orientations in AISI 304 stainless steel with

    3 µm grain